Stressed clams secrete a very smelly mucus. Cleaner shrimp are attracted to it incredibly! Vicious circle, clam stressed emits mucus which attracts shrimp. Attacked clam secretes more mucus and shrimp will, many times devour entire clam. Get some eggcrate (light diffuser) and screen off clam until mucus secretion stops. Just a thought. If/when shrimp no longer attracted to stressed clam, remove "cage"